Nonvolatile memory system and method of decentralizing the peak current in a nonvolatile memory system

ABSTRACT

A nonvolatile memory system has a controller chip connected to a memory medium and several nonvolatile memory chips. The memory medium stores program codes for the controller chip to distribute an operation of the nonvolatile memory chips upon an instruction over time, so as to decentralize the peak current caused by the operation and thereby improve the stability of the system.

FIELD OF THE INVENTION

The present invention is related generally to a nonvolatile memorysystem and, more particularly, to decentralization of the peak currentin a nonvolatile memory system.

BACKGROUND OF THE INVENTION

More and more applications have been found for nonvolatile memories suchas flash memories. For example, it is popular to use flash memories inmobile phones, digital cameras, personal digital assistants and portabledrives. Currently, memory card is one of the most popular products offlash memories, in which a controller chip is used to control thereceiving of data from a host such as a card reader, and the writing ofthe data into the flash memory. However, the memory card is designedwith more and more memory capacity, and to increase the memory capacityof a memory card, there are usually two solutions, one is to increasethe memory capacity of single flash memory chip, and the other is toincrease the number of flash memory chips in a memory card. For thelatter case, the power consumption during the operation of the flashmemories in a memory card becomes a thorny problem.

FIG. 1 shows the operational current when a flash memory chip operatesupon an erase instruction, and FIG. 2 shows the total operationalcurrent when two flash memory chips are simultaneously erased.Typically, a flash memory chip may require a peak current more than 100mA when it is erased. For example, in a memory card composed of acontroller chip with the serial no. SK6626AAPC and flash memory chipswith the serial no. TC58NVG3D1D, single flash memory chip requires anextra current between 125 mA and 133 mA when it is erased, as shown inFIG. 1. The extra current is referred to the difference between themaximum and the minimum of an operational current. As shown in FIG. 2,when two flash memory chips are simultaneously erased in a word mode,the extra current becomes about 230 mA to 239 mA. Briefly, the peakcurrent caused by the simultaneous operations of several flash memorychips in a conventional flash memory system is proportional to thenumber of the flash memory chips. A great peak current will bring thepower supply into unstable condition and thereby degrade the stabilityand reliability of the operation of the host, the controller chip andthe flash memory chip.

Conventionally, the solution for this peak current issue is directed tothe improvement of the hardware design of the memory system itself, soas to source the power for different components at different timepoints, which are conventionally provided power at a same time. Forexample, U.S. Pat. Nos. 7,085,189, 7,224,617 and 7,200,062 are sucharts. In U.S. Pat. No. 7,085,189, flash memory ships are divided intofour banks for data storage, among which three are added with delaycircuit such that the four memory banks will be erased at different timepoints upon an erase instruction, so as to reduce the peak current whenthe four memory banks are to be erased. U.S. Pat. No. 7,224,617 providesa high speed operation mode and a low current consumption mode for aflash memory system, and in the high speed operation mode, if the peakcurrent causes the flash memory system and the host it is connectedunstable, the system will switch to the low current consumption mode.U.S. Pat. No. 7,200,062 sets different time delays for different DRAMchips when the system is to refresh the DRAM chips, so as todecentralize the peak current produced by the refresh operation.

However, prior arts all focus on improving the hardware design of thememory system to decentralize the intensity and the occurrence time ofthe peak current, which needs to change the hardware design and is lessflexibility. Therefore, an alternative approach which does not requireto change the hardware design of a nonvolatile memory system todecentralize the peak current thereof is desired.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a nonvolatile memorysystem and control method thereof, to decentralize the peak current whenthe nonvolatile memory system operates upon an instruction.

Another object of the present invention is to provide a memory mediumstoring program codes for a nonvolatile memory system to decentralizethe peak current when it is in an operation.

A nonvolatile memory system according to the present invention comprisesa controller chip connected to several nonvolatile memory chips and amemory medium. The memory medium stores program codes, and thecontroller chip decentralizes an operation according to the programcodes when it is to operate the nonvolatile memory chips upon aninstruction. Preferably, in an operation of the nonvolatile memory chipsupon an instruction, the controller chip will send a first command to afirst one of the nonvolatile memory chips, and wait for a period of timebefore further sending a second command to a second one of thenonvolatile memory chips. Alternatively, in an operation of thenonvolatile memory chips upon an instruction, the controller chip willsend a first command to a first one of the nonvolatile memory chips, andfurther send a second command to a second one of the nonvolatile memorychips after receiving a response signal from the first nonvolatilememory chip. Therefore, the nonvolatile memory chips to be operated bythe controller chip upon an instruction will not all operate at a sametime.

By distributing the operation of the nonvolatile memory chips upon aninstruction over time, especially sending commands to each of thenonvolatile memory chips at different time points, the peak current ofthe entire system is reduced. Furthermore, this improvement is made bysoftware approach, and thus does not need any change or modification tothe hardware architecture.

BRIEF DESCRIPTION OF DRAWINGS

These and other objects, features and advantages of the presentinvention will become apparent to those skilled in the art uponconsideration of the following description of the preferred embodimentsof the present invention taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a waveform diagram showing the operational current when aflash memory chip is erased;

FIG. 2 is a waveform diagram showing the operational current when twoflash memory chips are simultaneously erased;

FIG. 3 shows a timing correspondence between an instruction executionand the operational current it causes in a conventional nonvolatilememory system;

FIG. 4 shows a timing correspondence between an instruction executionand the operational current it causes in a nonvolatile memory systemaccording to the present invention;

FIG. 5 is an embodiment of a nonvolatile memory system according to thepresent invention;

FIG. 6 is a flowchart for the operation of the controller chip shown inFIG. 5; and

FIG. 7 is a waveform diagram showing the operational current in asimulation when a method according to the present invention is used in anonvolatile memory system.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 shows a timing correspondence between an instruction executionand the operational current it causes in a conventional nonvolatilememory system, in which waveform 10 represents the erase signal sent toa first nonvolatile memory chip, waveform 12 represents the erase signalsent to a second nonvolatile memory chip, and waveform 14 represents theoperational current of the entire nonvolatile memory system. Forexample, in a conventional nonvolatile memory system including severalflash memory chips, to erase the flash memory chips, the controller chipwill sequentially send a command A and a command B to each flash memorychip, as shown by the waveforms 10 and 12 of FIG. 3. Typically the spacetime between the commands A and B is loose, and each flash memory chipwill execute the erase operation only when it receives the command A andthereafter the command B. In a conventional system, a command B is sentto each memory chip at a same time, and therefore, almost all of theflash memory chips will receive the command B at a same time and executethe erase operation simultaneously. As a result, the peak current forthis erase instruction is centralized at a time point, as shown by thewaveform 14 of FIG. 3.

Since the peak current caused by the operation of several nonvolatilememory chips upon an instruction at a same time is proportional to thenumber of the nonvolatile memory chips, it may reduce the peak currentby operating the nonvolatile memory chips at different time points. FIG.4 shows a timing correspondence between an instruction execution and theoperational current it causes in a nonvolatile memory system accordingto the present invention, in which waveform 16 represents the erasesignal sent to a first nonvolatile memory chip, waveform 18 representsthe erase signal sent to a second nonvolatile memory chip, and waveform20 represents the operational current of the entire nonvolatile memorysystem. For example, a command B is sent to a first nonvolatile memorychip, and after a period of time, a command B is sent to a secondnonvolatile memory chip, as shown by the waveforms 16 and 18 of FIG. 4.Since the two nonvolatile memory chips receive their commands B's atdifferent time points, they will execute erase operations at differenttime points, even though the erase operations are triggered by a sameinstruction. If the space time between the commands B's that are sent tothe two nonvolatile memory chips is longer enough, the operationalcurrent caused by operating the two nonvolatile memory chips will beseparated to have two peaks at two time points, and the peak currentwill be significantly reduced, as shown by the waveform 20 of FIG. 4. Inother words, decentralizing the operation time of several flash memorychips such that at any moment there are less flash memory chipsexecuting a command will distribute the peak current of the entiresystem over time, thereby reducing the peak current and improving thestabilities of the nonvolatile memory system and the host it isconnected.

FIG. 5 shows a nonvolatile memory system 30 according to the presentinvention, in which a memory medium 32 is connected to the controllerchip 34 and stores program codes to be accessed by the controller chip34, and two flash memory chips 36 and 38 are connected to the controllerchip. 34 and could be operated by the controller chip 34. The memorymedium 32 may be a firmware, for example a Read-Only Memory (ROM). Whenthe controller chip 34 is to operate the flash memory chips 36 and 38upon an instruction such as an erase instruction, it will distribute theoperation of the flash memory chips 36 and 38 over time according to theprogram codes in the memory medium 32. For further detail, FIG. 6provides a flowchart of the process that the controller chip 34 operatesthe flash memory chips 36 and 38 upon an instruction. Referring to FIGS.5 and 6, when the controller chip 34 receives an instruction, forexample one to erase the flash memory chips 36 and 38, so as to operatethe flash memory chips 36 and 38, it will send a command CMD1 to theflash memory chips 36 first, as shown in Step S40, and then wait forresponse from the flash memory chips 36 first, as shown in Step S42.After the flash memory chip 36 receives the command CMD1, it will send aresponse signal Sri to the controller chip 34. As shown in Step S44, thecontroller chip 34 will send a command CMD2 to the flash memory chips 38after it receives the response signal Sr1. Then, the controller chip 34waits for response from the flash memory chips 38 in Step S46. After theflash memory chip 38 receives the command CMD2, it will send a responsesignal Sr2 to the controller chip 34. After the controller chip 34receives the response signal Sr2, the instruction to operate the flashmemory chips 36 and 38 is to be completed.

FIG. 7 shows the operational current in a simulation when a methodaccording to the present invention is used in a memory card composed ofa controller chip with the serial no. SK6626 and two flash memory chipswith the serial no. TC58NVG3D1D. In this simulation, upon aninstruction, the two flash memory chips execute erase operationsinitiated at different time points, and thereby produce two peaks in theentire system current as shown in FIG. 7. The simulation result showsthat the extra currents for each erase operation of the flash memorychips are not over 150 mA. The efficiency reduction caused by the methodof decentralizing the peak current according to the present invention isnegligible. As shown in FIG. 7, the space time between the two peaks isabout 1.3 μs, and a flash memory chip typically needs about 1 ms-3 ms tocomplete an erase operation. If executing erase operation needs 1 ms, itonly causes 0.13% efficiency reduction in this simulation.

While the present invention has been described in conjunction withpreferred embodiments thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and scopethereof as set forth in the appended claims.

1. A nonvolatile memory system comprising: a controller chip; aplurality of nonvolatile memory chips, each connected to the controllerchip; and a memory medium connected to the controller chip, havingprogram codes stored therein; wherein the controller chip willdecentralize an operation of the plurality of nonvolatile memory chipsover time according to the program codes when it receives an instructionto operate the plurality of nonvolatile memory chips, so as to reducethe peak current caused by the operation.
 2. The nonvolatile memorysystem of claim 1, wherein the instruction comprises to erase theplurality of nonvolatile memory chips.
 3. The nonvolatile memory systemof claim 1, wherein according to the program codes, the controller chipsends a first command to a first one of the plurality of nonvolatilememory chips and after a period of time, sends a second command to asecond one of the plurality of nonvolatile memory chips.
 4. Thenonvolatile memory system of claim 1, wherein according to the programcodes, the controller chip sends a first command to a first one of theplurality of nonvolatile memory chips, and after receiving a responsesignal from the first nonvolatile memory chip, sends a second command toa second one of the plurality of nonvolatile memory chips.
 5. Thenonvolatile memory system of claim 1, wherein the memory mediumcomprises a ROM.
 6. A computer-readable memory medium storing programcodes for a controller chip to operate a plurality of nonvolatile memorychips upon an instruction, comprising the steps of: sending a firstcommand to a first one of the plurality of nonvolatile memory chips; andafter a period of time, sending a second command to a second one of theplurality of nonvolatile memory chips.
 7. The computer-readable memorymedium of claim 6, wherein each of the first and second commands is anerase command.
 8. A computer-readable memory medium storing programcodes for a controller chip to operate a plurality of nonvolatile memorychips upon an instruction, comprising the steps of: sending a firstcommand to a first one of the plurality of nonvolatile memory chips; andafter receiving a response signal from the first one of the plurality ofnonvolatile memory chips, sending a second command to a second one ofthe plurality of nonvolatile memory chips.
 9. The computer-readablememory medium of claim 8, wherein each of the first and second commandsis an erase command.
 10. A method of decentralizing the peak current ina nonvolatile memory system when operating a plurality of nonvolatilememory chips of the nonvolatile memory system upon an instruction, themethod comprising the steps of: sending a first command to a first oneof the plurality of nonvolatile memory chips; and after a period oftime, sending a second command to a second one of the plurality ofnonvolatile memory chips.
 11. The method of claim 10, wherein each ofthe first and second commands is an erase command.
 12. A method ofdecentralizing the peak current in a nonvolatile memory system whenoperating a plurality of nonvolatile memory chips of the nonvolatilememory system upon an instruction, the method comprising the steps of:sending a first command to a first one of the plurality of nonvolatilememory chips; and after receiving a response signal from the first oneof the plurality of nonvolatile memory chips, sending a second commandto a second one of the plurality of nonvolatile memory chips.
 13. Themethod of claim 12, wherein each of the first and second commands is anerase command.